The use of orthodontic appliances, commonly known as "braces," for achieving proper alignment of teeth has been well known and practiced for many years. The realignment of teeth through orthodontic practices is generally desirable to assure a proper "bite" by an individual to assure that food is properly chewed before being passed to the digestive system. Further, such realignment of teeth has been found to be most desirable for aesthetic reasons.
Previously, metal brackets have been secured to the teeth by means of bands or the like. In recent years, however, direct bonding of the brackets or orthodontic appliances onto the dentition have revolutionized the orthodontic practice. Direct bonding offers several advantages over conventional orthodontic bands. Among these are improved aesthetics, improved hygiene and gingival health, ease of manipulation and placement, and reduced decalcification. The invention herein relates to such direct bonding techniques. Direct bonding typically requires an etching of the tooth enamel to change the surface properties of the enamel from low energy hydrophobic to high energy hydrophilic which increases the surface tension and wetability. Bonding of adhesive composite resins to the etched enamel is achieved by mechanical interlocking with the etched surface.
The most commonly used bonding adhesives for orthodontics are acrylate and diacrylate resins such as Bis GMA. These adhesive systems consist of two parts; a low viscosity unfilled sealing resin and a highly filled paste. Sealing resins are used for a variety of reasons, but, primarily to facilitate wetting and penetration of the etched enamel surface, and as a coupling agent to provide chemical union between the surface and highly filled paste. Sealant resins prevent marginal leakage which can cause bond failure, and they prevent decalcification around the periphery of the bracket base.
The adhesive pastes have an inorganic filler content which varies in the range of 60-80% in weight. Conventional filling materials have quartz particles that lack uniformity in size and shape, or recently, marble shaped silica glass beads having a diameter of approximately 5 microns. The glass fillers are specially treated with silane. Three silane coupling agents are currently in use: gamma-aminopropyltriethoxysilane, vinyltriethoxysilane, and gamma-methacryloxypropyltrimethoxysilane. The latter is most commonly used. An important feature of the silane coating on the glass fillers is to the facilitate bonding between the glass filler particles and the organic matrix of the composite to give the material strength.
A dental composite is a combination of at least two chemically different materials in which a large amount of inorganic filler has been added to the resin matrix. Highly filled diacrylate resins have been shown to produce superior bond strengths. Curing of composite resins can be accomplished chemically with tertiary amine-benzoyl peroxide, or by using a light source to form cross-linked thermoset resin. Chemically cured composites have two adhesive pastes. When the catalyst and base pastes are mixed, polymerization results in cross-linkage, creating a three-dimensional combination that has increased strength, hardness, and dimensional stability. Ultra-violet light has been used to cure filled cross-linked thermoset diacrylate resin. However, such light is poorly transmitted by the tooth structure and requires a long exposure to cure the resin adhesive. Harmful side affects from such radiation is well documented. Recently, a catalyst system for polymerization of diacrylate resin which depends on visible light for activation has been developed. The wave length for visible light curing is in the range of 440-480 nm and does not present any potentially harmful side affects. It is well transmitted by the tooth and, consequently, the exposure time for curing the composite has been dramatically reduced to around 40 seconds.
Chemically cured composite systems have several disadvantages. A major drawback is the inability to manipulate the setting time. Polymerization begins immediately upon mixing and the material sets rapidly once mixing is complete. The working time is restricted and this limits the number of brackets that can be placed with one mix. Mechanical mixing of the two pastes often traps air in the composite resin which inhibits polymerization and concentrates stress that can further weaken the adhesive bond. When the bracket is placed, excess composite is often difficult to remove. Plaque readily accumulates on the excess composite, contributing to gingival irritation and possible enamel decalcification.
Visible light cured composites offer several advantages over the conventional chemically cured adhesives. A major advantage is the command set. The setting time can be controlled and the resin will only set when exposed to an intense white light source. Incorporation of air into the composite is less likely because there is no mixing involved. When using light activated materials, a slight excess of adhesive can be placed on the bracket base. When the bracket is seated, the excess composite extrudes and the possibility of voids at the edge of the bracket is reduced. The flash can easily be removed from around the bracket before curing is initiated. The viscosity of the light cured materials does not change and this makes manipulation predictable. Unlimited working time allows for ideal bracket positioning which is critical with straight wire appliances. Sealant can be placed on the base of the metal brackets to eliminate air from the retentive mechanism and allow for complete penetration of resin into the retentive areas.
The invention described hereinafter can be used with any type of orthodontal resin or adhesive, but the state of the art is directed to the use of visible light cured composites as just discussed. Accordingly, the invention will be described with respect to implementation with such composites, but without restriction or reservation to the implementation thereof with other bonding adhesives.
It is well known that interfacial and internal defects have a tendency to reduce bond strength. Air voids incorporated into the resin during mixing weaken the adhesive composite. Stress concentration may arise at these sites during polymerization of the resin, which may propagate along the defects during application of tensile and sheer stresses. Such weakening is well known and documented, as is the adverse affect of such air voids on polymerization.
The composite film thickness between the bracket and the tooth is an important factor which is often overlooked. A minimum layer of film is important for obtaining a maximum adhesion. A thicker adhesive interface produces more imperfections, greater polymerization shrinkage, and may fracture more readily. Uneven resin thicknesses can lead to residual stress in the adhesive film. It has been found that as the polymerization gap decreases, shear bond strength increases in an exponential relationship. Controlling the film thickness during bonding is extremely important, but heretofore has been unachieved.
The bonding of metal orthodontic attachments is primarily through mechanical retention between the bracket base and adhesive interface. Since the inception of direct bonding, metal brackets have been the most popular because of their mechanical advantages and familiarity among practioners. Several base designs of metal brackets have been manufactured; of these, mesh and groove based designs are the most popular. It is now accepted that mesh metal brackets provide adequate retention.
New ceramic brackets are renowned for their hardness, thermal integrity, and resistance to chemical erosion. They offer a distinct aesthetic advantage over conventional stainless steel brackets. However, ceramics have low fracture toughness and are therefore more prone to fracture. Typically, ceramic brackets are made of aluminum oxide (alumina). They are chemically inert and do not directly adhere to bonding adhesives. In order to solve this problem, ceramic bracket bases are available with two retentive mechanisms. The bases are either treated with a coupling agent that will chemically bond to the composite resin, or they have a combination of chemical coupling with mechanical undercuts for retention. The ceramic bases are typically treated with a silane coupling agent similar to the bonding agent applied to the glass fillers to provide for coupling between the glass fillers and the organic matrix of the adhesive composite.
While there are various types of orthodontic brackets presently used, metal or ceramic, the invention herein is adapted for implementation with either, having particular applicability to the ceramic brackets which are presently the trend in the art.
With reference now to FIG. 1 the prior art structure for adhering a bracket to a tooth may be seen. As shown, a bracket and tooth assembly is designated generally by the numeral 10. The bracket 12 whether metallic or ceramic, is adhered by means of an adhesive composite 14 to the enamel layer 16 of the tooth 18. The glass silane-coated particles 20 are interposed in the adhesive composite 14 to enhance bonding strength. Of course, metal wires or other tensioning means are interconnected between the grooves of the brackets 12 from one tooth to another to obtain the desired movement and alignment of the teeth.
Once the orthodontic treatment requiring the "braces" and requisite brackets 12 has been completed, it is important that the brackets 12 be readily removed from the teeth 18 without damage to the enamel surface 16. Typical debracketing instruments remove the appliance by application of both shear and tensile stresses. The bracket is removed, yet, considerable composite still remains on the tooth. Tensile forces transmit a significant amount of stress to the enamel layers 16 and are not recommended. If crazing lines are present in the enamel, tensile debonding forces are quite likely to pull enamel off with the bracket if fracture is at the enamel-bracket interface.
The issue of debonding ceramic brackets has raised a great deal of concern. Silane treatment of the ceramic base produces exceptional bond strengths which exceed the fracture toughness of the material. Debonding stresses can be shifted from the bracket/composite interface to the composite/enamel interface. Adhesive bond failures at the composite/tooth interface have been reported to be potentially harmful. Cohesive failure of the enamel is also not desirable. Ceramic brackets are likely to fail at these locations. Rigid, brittle ceramics have little ability to absorb stresses. Since the bracket/adhesive bond is dramatically enhanced by virtue of silane coupling, failure will usually occur in the ceramic, within the adhesive, or in the enamel. A sudden impact load is more likely to cause failure within the more brittle ceramic and enamel than in the polymeric bonding material 14.
Squeezing the tie wings of a ceramic bracket with a pair of pliers will undoubtedly cause ceramic failure. Similarly, use of a lift-off bracketing instrument will result in tensile failure of the fragile tie wings. Only diamond and pure carbon crystals are hard enough to cut the aluminum oxide bracket. If a portion of the bracket remains on the tooth after the debonding operation, it must then be ground off with a grinding wheel of diamond or the like, the same being a delicate operation, generating significant heat, and risking tooth damage. Further, ceramic brackets are not conducive to electrothermal debracketing techniques.
Ceramic brackets are frequently removed with a shear force applied by wedging between the enamel surface and bracket base with the blades of debonding pliers or ligature cutters. This technique leaves the least amount of residual adhesive on the tooth, however, significant enamel damage has been documented with this type of instrumentation. Bracket fractures are prevalent. Harm can result if one aspirates or swallows a ceramic fragment.
Because of the debonding problems associated with ceramic brackets, many practioners are reluctant to use them in their practices. Reports of enamel damage during the course of treatment and enamel failures while debonding are becoming prevalent. An improvement in the bonding of the ceramic brackets to the tooth enamel is required.